1. Field of the Invention
The present invention relates to a masking device, a related lithographic projection apparatus, and a device manufacturing method.
2. Description of the Related Art
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to impart an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                a mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table/holder, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        a programmable mirror array: one example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation mechanism. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from United States Patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        a programmable LCD array: an example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table/holder; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth here above.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask table/holders). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed 27 February, 1998 (WO 98/40791), incorporated herein by reference.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table/holder, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table/holder is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
It is often desirable or necessary to ensure that only a certain part of the mask is imaged by the projection beam to the substrate. For example, the mask may contain more than one pattern of which only one is used for a given exposure. It is also often desirable or necessary to stop stray light from impinging on the substrate. In lithographic projection systems using this function, it is typically achieved by providing a mask masking device at an intermediate plane in the illuminator.
In a stepper system, where the mask is fixed with respect to the illuminator, the mask masking device is provided adjacent the mask and is also fixed with respect to the illuminator and the mask.
Conventional masking devices comprise at least one movable blade. In certain conventional devices two sets of moveable blades are provided. Typically, the two sets of blades are mechanically coupled to a support and each support is mounted on a common frame. Thus, conventionally the sets of blades are mechanically coupled to each other.
Generally, the two sets of blades may be disposed in a plane in the illumination unit. Each set of moveable blades comprise a pair of blades arranged to move together and apart in one direction, the Y-direction, hereinafter referred to as the Y-blades, and the other pair of blades is arranged to move together and apart in a direction perpendicular to the Y-blades, in the X-direction, hereinafter referred to as the X-blades. There are currently two types of mask masking schemes: (a) masking for static exposure and (b) masking for scanning exposure. In static exposure, part of the mask is blocked for the duration of an exposure. In scanning exposure, a part of the mask is blocked for a predetermined length of time.
The blades may be set so that there is a predetermined distance between the X-blades and the Y-blades, respectively. Conventionally, the Y-blades are arranged to be moveable during scanning, and the X-blades, although moveable, are generally arranged to be stationary during scanning. If the X-blades are to be moved, this generally takes place in between scans. For static exposures the X-blades may be moved between exposures. For scanning exposure, Y-blades in particular are arranged to perform additional movements to allow scanning of the patterning device by the radiation source to take place. Before a scanning cycle begins, the blades are arranged to prevent any radiation impinging on the patterning device. At the beginning of the scanning cycle the Y-blades open to a scanning distance. At the end of the scanning cycle, the Y-blades, in particular, move into a position in which light is prevented from impinging on the patterning device, so that at the end of the scanning cycle no light impinges on the patterning device.